My client has multiple domains running through the same WordPress site. Suddenly they are no longer able to upload PDFs to their posts or their media library. They do not get an error that WP is unable to create a directory (as many other posts on the same subject have gotten), but they get 'the uploaded file could not be moved to (filepath)'. My workaround is to have them FTP the files to the desired location, then hand-code the link to that file. Obviously, this will not work for long.
I've set the permissions for those folders at 777 as well as the blogs.dir folder as I had seen suggested on another post dealing with the same issue.